There is a growing interest in new optical devices having an improved efficiency in terms of energy consumption, colour mixing and illumination uniformity. Collimators are known to be used in many illumination applications to produce a desired illumination pattern beam angle. However, many collimators tend to reproduce at least part of the spatial structure of the light source. Non-uniform light sources such that e.g. light emitting diode (LED) arrays and LEDs of different colours may hence cause artefacts visible in the resulting light beam and beam spot when combined with a collimator.
Existing optical devices may therefore comprise mixing means, such as a lenslet arrangement, which is added to the collimator so as to further mix the light and to reduce artefacts caused by the light source. WO 2012/107097 shows an example of such lenslet arrangement, wherein two nearly mirror symmetric lenslet structures are provided for improving the mixing of coloured light emitted from LEDs of different colour. The lenslet structures are arranged above each other in a vertical direction, i.e. the direction of the emitted light, and slightly offset to each other in a lateral direction to reduce any artefactual irregularities in the light beam.
Although such a lenslet arrangement may provide mixed light, there is a need for alternative optical devices having enhanced light intensity and allowing for an emitted light beam having a more narrow beam angle.
U.S. 2011/0188242 discloses an optical component in which light from light sources is collimated by a collimator and transmitted to the lens with a first and a second lenslet structure, the light being focused by the first lenslet structure on the second lenslet structure.
In “Köhler integrators embedded into illumination optics add functionality” by O. Dross et al (Proceedings of SPIE, Vol. 7103, 2008) an overview is given of the Köhler illumination concept and its application to different kind of optics.